Are you by any chance traveling with a child under the age of 12? On Thai's new A380's they do not allow children on the upper deck level, only if absolutely necessary they will. I applaud them for implementing this as this way business class will never be disturbed by a potential crying baby immediately behind them if they were to be seated in the last row of C class.
Or if you are *G or *P I'm sure if you notify them of your status they will kindly open it up for you.